Problem: Which of these numbers is composite? ${17,\ 23,\ 73,\ 79,\ 86}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 17, 23, 73, and 79 each have only two factors. The factors of 86 are 1, 2, 43, and 86. Thus, 86 is the composite number.